1. Field of the Invention
The present invention relates to organic EL displays and manufacturing methods thereof.
2. Description of the Related Art
In recent years, liquid crystal displays have been frequently used as flat-panel displays in a wide variety of fields. However, liquid crystal displays in which contrast and color significantly vary depending on the viewing angles and which need a backlight as a light source, still have critical problems, such as the difficulty in reducing power consumption and the limits in reducing thickness and weight. Therefore, self-luminous displays using, e.g., organic electroluminescence (hereinafter referred to as “organic EL”) have recently been expected to be substitutes for liquid crystal displays. In an organic EL device, upon passage of a current through an organic EL layer sandwiched between positive and negative electrodes, organic molecules in the organic EL layer emit light. Organic EL displays using organic EL devices are self-luminous, and thus have advantages in achieving smaller thickness, lighter weight, and lower power consumption. In addition, organic EL displays using organic EL devices have wide viewing angles, and therefore, are drawing considerable attention as probable next-generation flat-panel displays. Such organic EL displays, making use of their small thickness and wide viewing angles, are actually finding increasing practical application as displays for portable music devices and cellular phones.
In general, organic EL devices are manufactured by different methods depending on the type of functional material used therein. For example, when high molecular weight organic compounds are used as a functional material, the functional material is formed into a film by a wet method, such as a spin coating process, a screen deposition process, or an inkjet system. When low molecular weight organic compounds are used as the functional material, a dry method, such as a vacuum evaporation process or a sputtering process, is often used to form a film from the functional material.
Among the film forming techniques mentioned above, the inkjet system has excellent advantages; for example, the inkjet system in general allows more efficient use of film-forming material than the other methods, enables patterning without the need for a mask, and is readily applicable to manufacture of large-area organic EL displays. Therefore, manufacturing of organic EL devices using the inkjet system has been studied actively.
For example, Japanese Laid-Open Publication No. 2000-106278 describes the use of an inkjet type recording head in manufacturing an organic EL device that includes, between electrodes, hole injection transport layers containing conductive molecules and light emitting layers containing luminescent organic molecules. The inkjet type recording head is used to form the hole injection transport layers and the light emitting layers.
In the inkjet system, a barrier (a bank) for retaining discharged droplets within pixel areas is typically formed. This bank often has liquid repellency to prevent discharged droplets from leaking out into adjacent pixel areas. For example, Japanese Patent Publication No. 3328297 discloses a method in which the non-affinity (liquid repellency) of a bank is increased as compared with electrode surfaces within pixel areas by performing plasma processing using a gas containing fluorine compound or the like, thereby confining droplets within the pixel areas. Without such plasma processing performed in Japanese Patent Publication No. 3328297, it is also possible to render a bank repellent to droplets by forming the bank using a material containing fluorine compounds, for example. Furthermore, if the pixel areas located inside the bank are rendered lyophilic by UV processing or by plasma processing using an oxygen gas or the like, the difference in affinity for droplets between the bank and the inside of the pixel areas will increase, allowing more reliable retention of droplets within the pixel areas.
Japanese Laid-Open Publication No. 2004-6362 describes that if hole transport layers, electron transport layers, and negative electrodes are formed so as to extend through multiple pixels, it is possible to eliminate the need for the process step of etching back the hole transport layers and the electron transport layers, and the process step of forming a barrier. Japanese Laid-Open Publication No. 2004-6362 also describes that light emitting layers, formed in the same shape as, but larger than, positive electrodes, can prevent leakage current and concentration of electric field occurring in the edges of the positive electrodes. In Japanese Laid-Open Publication No. 2004-6362, it is assumed that the hole transport layers and the electron transport layers have high resistance, and based on this assumption, short-circuits between the positive and negative electrodes are prevented.
Carrier transport layers in an organic EL device typically have lower resistance and higher conductivity than light emitting layers so that a sufficient electric field is applied to the light emitting layers. In some cases, to prevent carriers from passing through the light emitting layers without contributing to light emission, a functional material layer, called a carrier blocking layer, which confines carriers within the light emitting layers, is formed between the carrier transport layers and the light emitting layers. However, such carrier blocking layers typically have low conductivity, and therefore, are often formed so as to be much thinner than the light emitting layers.
Now, it is assumed that, using an inkjet system, a hole transport layer, a light emitting layer, and a negative electrode are applied and stacked in that order on a pixel surrounded by a bank. Droplets containing a functional material for forming the hole transport layer are discharged inside the bank, and then dried and heated to remove the solvent, thereby forming the hole transport layer in the pixel. The next step is to discharge droplets containing a functional material for forming the light emitting layer on that layer. In this step, if the lyophilic property of the hole transport layer with respect to the light emitting layer formed thereon is insufficient, or if the droplets for forming the light emitting layer have excessively high surface tension, or even if neither of these is the case, depending on the shape of the bank, the droplets may not be retained inside the bank so as to completely cover the hole transport layer and the positive electrode. In that case, the light emitting layer formed through drying and heating processes also does not completely cover the hole transport layer and the positive electrode, causing the hole transport layer to be partially exposed. In this state, if a negative electrode is formed on those layers by a vacuum evaporation process or the like, then the exposed areas of the hole transport layer and positive electrode will make direct contact with the negative electrode without interposition of the light emitting layer. These contact areas do not contribute to light emission when current is passed through each pixel of the organic EL device for light emission. In addition, in those contact areas, current loss causes a decrease in intensity, and leakage current causes generation of heat, an increase in power consumption, etc., resulting in a serious problem in power efficiency and in device lifetime. Therefore, it is necessary that the light emitting layer completely cover the hole transport layer and the positive electrode. To completely cover the hole transport layer and the positive electrode with the light emitting layer, the amount of each droplet for forming the light emitting layer could be possibly increased. However, if an excessive amount of droplet is discharged in the pixel, the droplet might spill over the bank so as to flow out into unintended adjacent pixels. Moreover, such an excessive amount of the droplet also causes a problem in that a desired thickness cannot be achieved.
In a case where the hole transport layer is not used, and an electron transport layer is stacked on the light emitting layer instead, the result is the same. If the positive electrode is not completely covered with the light emitting layer, and is partially exposed, the electron transport layer makes direct contact with the positive electrode without interposition of the light emitting layer. As a result, in the contact areas, current loss causes a decrease in intensity, and leakage current causes generation of heat and an increase in power consumption, because the electron transport layer has lower resistance than the light emitting layer.
Also, in a case where the hole transport layer and the electrode transport layer are both used, the above-described problems also occur due to contact made, without interposition of the light emitting layer, between the positive and negative electrodes, between the hole transport layer and the electron transport layer, between the hole transport layer and the negative electrode, and between the electron transport layer and the positive electrode.
In the case of the materials used by the present inventors, hole transport layers and electron transport layers have lower resistance than light emitting layers. With such materials, it is not possible to prevent short-circuits between the positive and negative electrodes in the structure of Japanese Laid-Open Publication No. 2004-6362. In addition, as set forth above, it is desired that a voltage drop in carrier transport layers be as small as possible, that is, the resistance of the carrier transport layers be minimized, in order to apply a sufficient electric field to the light emitting layers.
A barrier is provided in Japanese Laid-Open Publication No. 2004-6362 in order to provide insulation between the positive and negative electrodes, and to prevent light emitting layers of different colors in adjacent pixels from mixing with each other to thereby cause a reduction in color purity. Therefore, it is difficult to eliminate the need for the process step of forming the barrier that is provided to retain droplets in respective pixel areas in a coating method, such as an inkjet system, for example.